London contains many prestigious and historic buildings that have an old fire alarm installed. Many of these systems wired in the old fire alarm cable at that time MICC or also known as Pyro, are well known to fail after an extended period of time, usually suffering from earth faults.
Furthermore, many of these types of buildings may not have automatic detection; however, with the introduction of the Fire Safety Order in 2005, a Fire Risk Assessment will probably state that automatic fire detection is now required to protect the occupants of the building and the building itself.
In this type of building it is not practical, or ascetically pleasing to install a wired system or to extend an existing system to accommodate the additional equipment.
Another obstacle for many buildings of this type is they may well be Grade Listed. Installing a fire alarm system in a Listed building may require the permission of the planning department, as any work that may affect the look or the character of the building will need planning permission.
Furthermore, you will usually find Listed Buildings have limited areas to hide the cables, and the walls are very thick, making drilling holes without damaging the fabric difficult.
However, for many, the easy answer is to install a wireless fire alarm system as shown in this video.
The latest wireless systems are very liable due to the enforcement of the wireless fire alarm equipment code EN54-25.